This invention relates to optical objectives for photographic devices, and more particularly to a telephoto objective comprising a stationary front lens group and a rear lens group which includes a member axially movable to focus while still maintaining improved stabilization of the aberrations throughout the focusing range.
In general, the focusing of an optical objective is effected by axially moving the objective bodily with reference to the image plane. But this may be otherwise achieved by axial movement of a part of the objective, for example, either the front, or the rear lens group thereof with reference to the other part which remains stationary during focusing. In telephoto objectives, if made bodily movable for focusing purposes, the amount of movement of the objective throughout the focusing range tends to increase, and this in turn calls for increases in the weight and bulk of the objective and the driving torque of its focusing mechanism as well as in the production cost thereof, thus it being made more difficult to keep the weight and bulk of the complete objective including its mechanical mounting and focusing mechanism within easily manageable proportion.
With a telephoto objective having the provision for focusing made at a rear part of the lens system, it is possible to minimize the movement of the focusing part and to employ a focusing mechanism of small size and weaker driving torque, because the focusing part comprises only a fraction of the weight and bulk of the complete objective, thereby giving an advantage of facilitating high-speed focus adjustment of the objective. Another advantage deriving from the selection of the rear lens group of the objective for employment as the movable part for focusing purposes is that the front lens group which is of a relatively large weight and bulk may be supported in fixedly secured relation with a camera body to minimize the probability of producing a jiggle or oscillation of the image at the focal plane by small accidental motions of the objective and camera assembly which is otherwise encountered particularly when the objective is telephoto in nature. However, this system has a disadvantage that variation of the aberration particularly variation of the spherical aberration due to the movement of the lens is large, and the image forming ability is low. Further, in order for an objective to embody an advantageously small overall length, a convergent lens group is usually selected for employment as the front part thereof in combination with a divergent rear lens group having design parameters determined so as to cancell out the residual spherical aberration introduced by the front lens group in a focusing position for an object at infinity. However, as the objective is focused from this position to a position for a close object, the axial separation between the front and rear lens groups is increased with decrease in the hight of incidence of an axial ray upon the rear lens group, so that occurrence of the positive spherical aberration is less, while varying the value of the spherical aberration to a remarkable extent. In this respect, it is well known that this arrangement is disadvantageous for stabilization of the image aberrations throughout the focusing range.